Chemical method for concentrating isotopes of carbon



June 13, 1950 M. CALVIN EI'AL 2,511,667

CHEMICAL METHOD FOR CONCENTRATING ISOTOPES OF CARBON Filed Aug. 13, 1948 INVENTORS MEL w/v CAL 1///v BY PETER E. K4NKW/CH A 7'7'ORNEY.

Patented June 13, 1950 W1 CE -CHEMICAL mernontomcowosnrrm'rmo "ISOTOPES OF TCARBON Melvin-1 Galvin andzPeter EJYankwieMBerkeley, Galif.;assignors to the United-States of America l as re presen tedby the United StateswAtomic EnergyICommis'sion i lkpplicationAugu'st 13, 19 18,? Sefiittl'NmMliDl-B i"isotopes "f uniquestionable scientific importancel Rea'ctions'maybe studied in' detail and structures "which'were heretofore Euncertain can now be positivelyestablished by means of these isotopes.

Since thework donewith isotopesatthepresent tl-me is largely in the field of organic chemismy; is tobe 'expected that large concentrations of -the isotopes of carbon would be'i-n demand throughout the 'entire chemical World. It is with -the isotopes of carb'on that the present invention is-i-'concerined and clearlyiepresents' an advance 'over the earlier metho'dsof isoto econcentration.

lln thepast vatious methodsforthe concentration-of isotopesl havembeen 'devlsed. The first 10f "thesewas ithe mass-spectroscopic methodwhich, whenzkiealmgi-iwith minute 'iquantities, F completely separated the isotopes from:- each other. Methods 0f: enrichment-followed rand these-included :separation by various iorms'of diff usion. Porous wall difiusionsgravitational difiusion; and thermal-dif- 'fu'sion have been used successfully. -M11ltip1e "stage diffu'sionaprocesses followed and-thesewere found 'to fibe orvalue for-particular elements. O"ther r'n'ethods I for 1 enrichment include electro- "chemical rm'eans, fractional distillation, and (Sheila-Rial exchange reactions. These methods-are *allyaluable "arid manifold applications in t-he-'prdduotion off isotopes' at the present time.

The chemical exchangereaction method for en- "*ilchment depends on the slight change in the "equilibrium constant d-uring a reaction involving one of "the ''elements in isotopic form. This "change in the equilibrium constant follows from the fact that there are di'fierences inthereactivities of isotopicmolecules; these differences being mainlyattributed to the residual or zero point energy. Catalysts have been developed for particular processes"of-"enrichment and. satisfactory results have been obtained particularly with certain specific reactions.

The present invention also makes use of a phys- 3 Claims. (citric-441 'ical iich'emical Ipropefit aof .n'isotopes 'of chemical #elements "but lthe I 'diffierences ezbetween ith'e two methoos 'be readily apparent from aam'exnation of-ithe'rdescription iandtclaimssfthereforei am 'object ofithe presentcinvenition' toflprovidea usefulppraotical chemicalmethiodl for the concentration oftheiisotopeszofz car- =bon.'

lt iis-lei furtherf objecteoftzthettpresentinvention 'to :pr'oi ide a fnew aind revolutionary rchemical "-method for' th-e concentration of the isotopesl-of "carbon.

It -is a still Tfm'therfiobject of z the 2 present 1nver'ition to provide a'newrchemical method for the concentration of the isotopes .:df' :-carbon which be readily adaptable to "commeroia'lfproduc- =tionr1e7nq -whichvwiil beioffrmarkedrsignificance' as "a fof vvand step: thexgreati'need:forthese particsolar isotopes intthe iiields of tracer chemistry r andlmedicine.

Other objects and advantages will be apparent flin the-lollowing des'criptiomand? the iaccomp'anying *drawin'g, tln'iwhich Figure i -is a'fiowlshe'et:tliagram ofs'a' particular 5 experimental run illustrating the necessary-'-steps to ob tainithedesired?enrichment by thetmethod hesein descrlbed and is the sole figure thereof This invention makes ruse :ofa' la'w of: physical ohem-istryowhich establishes "that: there-is :a dif- *teren'ee betweeni the bond strengths 'Of' 'a' C C linkagefiand 'ia C :G "link-'a'g e, where :Xrrepres'e'ritsr-zanfi-atomic nweight corresponding to that'of an isotope of carbon other than 12.

IIr order -forvthe' present invention .to Bbe' oper- Fame: ismecessary' toistartwith an organic-mate1 ial in which theF'f u'nctional groups iare sym- -meti'ioal. two icompounds: of this type are- =malo1iic acid Iand' pkithaliciacid. umy-or'all of the sy mnietrical functional igroups in -these com- 40 pounds may be labeled with isotopic carbonsthat is, detectably-amounts 'o'faa-ny ione' of :th'erisotopes of' "ce3rbon-i=ma n ='allowa'bly. be; present-in one or all of the cfiunctional groups.

Since' there'xisiazdifierencefinthe bond strengths as hereinbefore discussed, the partial decar-bo bond is stronger than the C C bond. Experimental values indicate, in the case of the partial (partial in this instance relates to the completion of the process X1RX2 X1+RX2 where X1 and X2 are chemically identical atoms or groups, i. e., COOI-l) decarboxylation of m-alonic acid labeledwith C that the most conservative value for a/b -'1.12i0.03. The factor a/b is the ratio of the frequencies with which each path is taken during the process of partial decarboxylation.

Since this relationship exists, the amount of isotopic (isotopic: an isotope in such relative" amount that it serves as a label, i. e., present in a concentration other than that ordinarily found v in "natural or normal carbon) carbon can be concentrated and the normal C -portions discarded by means of repeated partial decomposition and selected synthesis. This process of partial decomposition and selected synthesis is continued until the point at which the CO2 becomes enriched to such a degree that it may be fed back into the process at an earlier step in lieu of being discarded. This will become more apparent after a consideration of the following eX- ample and Figure 1. The example is described in progressive steps, each number and letter corresponding with a particular phase of the process as illustrated in Fig. 1. Example: Start with Cl-I2(C*OOH)2 labeled with 1% C The method and resulting concentrations are as follows:

(1) 100 moles of malonic acid with 1.0% C decomposes into 100 moles of acetic acid with 1.1% C and into 100 moles of CO2 with 0.9% C

1 This particular 100 moles of CO2 is withdrawn from the process and may be reserved as future starting material.

' (2) The 100 moles of acetic acid (1.1% C formed in step (1) above is divided into two portions (A) and (B) of 50 moles each. From portion (A) is formed, by decarboxylation, 50 moles of CO2 with 1.1% C Portion (B) is retained as the process and may be reserved as future start- 1 ing material.

(5) Step (2) is substantially repeated with the 50 moles of acetic acid (1.21% C) formed in step (4) as follows: the 50 moles of acetic acid is divided into two portions (A) and (B) of moles each. From portion (A) is formed 25 moles of CO2 with 1.21% C Portion (B) is retained as such.

(6) Step (3) is substantially repeated as follows: portion (B) of step (5) is combined with the 25 moles of CO2 formed from portion (A) of step (5) to form 25 moles of malonic acid with ('7) Step (1) is substantially repeated with the 25 moles of malonic acid formed in step (6) as follows: 25 moles of malonic acid' with 1.21% C decomposes into 25 moles of acetic acid with 1.33% C and into 25 moles of CO2 with approxi-, mately 1.1% C

(8) The CO2 with approximately 1.1% C is fed back as a substitute for part of the CO2 from part (A) of step (2) in either a continuous process or in a later repetition of the original process. This feeding back into either a continuous or later repeated process has the very desirable effect of eliminating the necessity of splitting the moles of acetic acid, resulting fromstep (l) into equal parts, (A) and (B)- Instead, part (B) of step (2) can thereby be increased because the amount of CO2 to be produced from part (A) of step (2), which is to be used in the synthesis of malonic acid in step (3) can now be substantially reduced; a portion of the required CO2 being replaced by the enriched CO2 formed from step (7) P The steps of decomposing and synthesizing recited in the above process may be repeated still further than indicated in Fig. 1, whereby a greater degree of enrichment of the specific carbon isotope is obtained.

It is to be noted from the foregoing that the process outlined above is adaptable to a continuous process for commercial production of carbon compound enriched with a particular isotone of carbon. i

While the salient features of this invention have been described in detail with respect to one embodiment, it will of coursebe apparent that numerous modifications may be made within the spirit and scope of this invention, and it is not therefore desired to limit the invention to the exact details shown except insofar as they may be defined in the following claims.

What is claimed is: 1. In a chemical method for concentrating a isotope of carbon, the steps comprising partially decarboxylating an organic compound selected from the group consisting of phthalic acid and. malonic acid in which at least one of the functional groups is labeled with isotopic carbon to form a product enriched with said isotope, reforming said organic compound employing said enriched product as the source of the synthesizing material, and repeating said steps to effect the desired degree of isotopic enrichment.

2. In a process for concentrating an isotope of carbon, the steps comprising decarboxylating a compound selected from the group consisting of phthalic and malonic acids with a labeled functional group to form a monocarboxylic acid, decarboxylating a portion of said monocarboxylic acid to yield labeled carbon dioxide, reforming said compound in an enriched form using a second portion of said monocarboxylic acid and said carbon dioxide, and repeating said steps to effect the desired degree of isotope enrichment.

3. In a multistage process for concentrating an;

isotope of carbon, the steps comprising decarboxylating a compound selected from the group consisting of phthalic and malonic acids having a labeled functional group to form a monocarboxylic acid and a portion of carbon dioxide, decarboxylating a portion of said monocarboxylic acid to yield another portion of carbon dioxide, reforming said compound using a second portion of said monocarboxylic acid and said other portion of carbon dioxide, and performing said steps in cyclic sequences with the reintroduction of said portions of carbon dioxide to effect a further enrichment of said isotope.

MELVIN CALVIN. PETER E. YANKWIC I-l.

No referencescited. 

1. IN A CHEMICAL METHOD FOR CONCENTRATING AN ISOTOPE OF CARBON, THE STEPS COMPRISING PARTIALLY DECARBOXYLATING AN ORGANIC COMPOUND SELECTED FROM THE GROUP CONSISTING OF PHTHALIC ACID AND MALONIC ACID IN WHICH AT LEAST ONE OF THE FUNCTIONAL GROUPS IS LABELED WITH ISOTOPIC CARBON TO FORM A PRODUCT ENRICHED WITH SAID ISOTOPE, REFORMING SAID ORGANIC COMPOUND EMPLOYING SAID ENRICHED PRODUCT AS THE SOURCE OF THE SYNTHESIZING MATERIAL, AND REPEATING SAID STEPS TO EFFECT THE DESIRED DEGREE OF ISOTOPIC ENRICHMENT. 